1. Field of the Invention
This invention relates to an intake system for an engine, and more particularly to an intake system for an engine which is arranged to supercharge the engine by the kinetic effect of intake air.
2. Description of the Prior Art
Various multiple-cylinder engines are known which are arranged to increase the charging efficiency, and thereby the engine output torque, by the kinetic effect of intake air such as an inertia effect or a resonance effect of intake air. For example, in the multiple-cylinder engine disclosed in Japanese Unexamined Patent Publication No. 62(1987)-121828, the cylinders are divided into two groups so that the cylinders in each group do not fire one after another, discrete passages for the cylinders in each group are connected to an enlarged volume chamber, the enlarged volume chambers for the respective cylinder groups are connected to the downstream ends of a pair of long resonance passages which are connected to each other at their upstream ends, and the enlarged volume chambers are connected by a short resonance passage which is provided with an on-off valve.
In such an engine, supercharging effect by the kinetic effect of intake air can be obtained over a wide engine speed range by opening and closing the on-off valve according to the operating condition of the engine. For example, when the on-off valve is closed in a low engine speed range, an air column between the intake port for each cylinder and the upstream end of the long resonance passage is oscillated by a negative pressure wave generated in the intake stroke of the cylinder and when the engine speed tunes to the natural frequency of the air column, the amplitude of the oscillation of the air column is maximized and a resonance supercharging effect is obtained, whereby the charging efficiency is increased. When the on-off valve is opened in a high engine speed range, an air column between the intake port for each cylinder and the enlarged volume chamber for the other cylinder group with which the cylinder is communicated by way of the short resonance passage is oscillated by said negative pressure wave and when the engine speed tunes to the natural frequency of the air column, the amplitude of the oscillation of the air column is maximized and another resonance supercharging effect is obtained, whereby the charging efficiency is increased. When the engine speed increases further higher, a negative pressure generated at the beginning of the intake stroke of each cylinder is reflected as a positive pressure wave at the enlarged volume chamber and propagates downstream and acts on the same cylinder at the end of the intake stroke to supercharge the cylinder by an inertia effect of the intake air. The inertia supercharging effect is maximized at a particular engine speed which is determined mainly according to each discrete passage.
However, the engine is disadvantageous in that the surge tanks add to the overall size of the intake system. Especially in the case of a V-type engine, the surge tanks must be disposed above the cylinder banks, and accordingly, the height of the engine increases. On the other hand, when the surge tanks are eliminated, the inertia supercharging effect deteriorates and the engine output power cannot be increased especially in the high engine speed range.
When an engine can be supercharged by the resonance effect of intake air, which needs no surge tank, at the high engine speed range, the overall size of the engine can be reduced. For such a purpose, an intake system shown in FIG. 9 can be conceived.
The intake system shown in FIG. 9 is for a V-6 engine which has first to sixth cylinders #1 to #6. The first, third and fifth cylinders #1, #3 and #5 are formed in a row in first cylinder bank 1A and the second, fourth and sixth cylinders #2, #4 and #6 are formed in a row in a second cylinder bank 1B. The firing order is 1-6-3-4-5-`, and accordingly, the cylinders in each cylinder bank do not fire one after another.
The first, third and fifth cylinders #1, #3 and #5 in the first cylinder bank 1A are communicated with a first communicating passage 3A respectively by way of first to third discrete intake passages 2.sub.1, 2.sub.2 and 2.sub.3, and the second, fourth and sixth cylinders #2, #4 and #6 in the second cylinder bank 1B are communicated with a second communicating passage 3B respectively by way of fourth to sixth discrete intake passages 2.sub.4, 2.sub.5 and 2.sub.6. The first communicating passage 3A extends substantially in parallel to the cylinder row in the first cylinder bank 1A, and the second communicating passage 3B extends substantially in parallel to the cylinder row in the second cylinder bank 1B. A main intake passage 6 branches at a junction 4 into first and second branch intake passages 5A and 5B which are substantially equal to each other in length. The downstream end of the first branch intake passage 5A is connected to the first communicating passage 3A at a portion opposed to the junction of the second discrete intake passage 22 to the first communicating passage 3A, and the downstream end of the second branch intake passage 5B is connected to the second communicating passage 3B at a portion opposed to the junction of the fifth discrete intake passage 2$ to the second communicating passage 3B.
First and second inter-cylinder-bank communicating passages 3C and 3D connect the upper ends (as seen in FIG. 9) of the first and second communicating passages 3A and 3B and the lower ends (as seen in FIG. 9) of the same, thereby forming a circular passage 3.
Generally, in the case of the resonance supercharging effect, the tuning engine speed at which the charging efficiency is maximized mainly depends on the mean cross-sectional area and the length of the part of the passage upstream of the portion at which the discrete intake passages in each cylinder bank communicate with each other. Accordingly, in the case of the intake system shown in FIG. 9, the circular passage 3 serves as a part of the passage upstream of the portion at which the discrete intake passages in each cylinder bank communicate with each other and the cross-sectional area of the upstream side portion increases, whereby resonance tuning engine speed becomes higher. Thus, the engine output power in the high engine speed range can be increased by the resonance supercharging effect.
However the intake system shown in FIG. 9 is disadvantageous in that since the first branch intake passage 5A opens to the first communicating passage 3A at a portion opposed to the second discrete intake passage 2.sub.2 and the second branch intake passage 5B opens to the second communicating passage 3B at a portion opposed to the fifth discrete intake passage 2.sub.5, intake air smoothly flows into the second and fifth discrete intake passages 2.sub.2 and 2.sub.5 but the passages through which intake air flows into the other discrete intake passages have crooks and intake air cannot smoothly flow into the other discrete intake passages. Accordingly, especially in the high engine range where the flow of intake is high, intake air cannot be uniformly distributed to all the cylinders and the charging amount of intake air varies from cylinder to cylinder.